In 1997, Michelin North America upgraded
the compressed air system controls at its tyre manufacturing
plant in Spartanburg, South Carolina. In response
to growing energy costs and the desire to remain competitive
in the tyre industry, Michelin performed an internal
evaluation of its compressed air system to determine
how it could improve the system's efficiency and energy
use. The evaluation provided the basis for a project
to install a new control system. The controls upgrade
project at Michelin's plant enabled multiple compressor
operation without blow-off during load swings. In
addition, the plant has been able to stabilize and
lower pressure levels, leading to estimated annual
energy savings of $75,000 (RM 285,000) and 2,143,000
kWh. The project's total cost was $120,000 (RM 456,000),
giving the plant a simple payback of approximately
a year and a half.
PLANT
BACKGROUND
Michelin North America is a branch
of the Compagnie Generale des Etablissements Michelin
(CGEM), a tyre manufacturer headquartered in Clermont-Ferrand,
France, that has 18 percent of the global tyre market.
The company is composed of fifteen tyre manufacturing
plants, two products plants that are partially finished,
and two textile plants.
The Spartanburg plant is a modern,
vertically integrated facility that employs approximately
1,000 people who produce truck tyres, steel belts,
and bead rings on rims for large transport trucks.
Compressed air directly supports the plant's production
process, which includes cylinders, air motors, and
conveyors. The plant's compressed air system is served
by five 500-horsepower (hp) centrifugal compressors.
Prior to the project, individual controls directed
each of the five compressors; these individual controls
were provided as standard equipment during installation
in the late 1970s and 1980s. Under this configuration,
the plant required all five compressors to operate
at a discharge pressure of 9 bar in order to adequately
supply the plant with the required air volume and
pressure.
PROJECT
OVERVIEW
An internal review of energy consumption
patterns at various Michelin North America facilities
revealed that a similar Michelin facility in Canada
had lower compressed air energy costs than the
Spartanburg plant. The Spartanburg plant engineers
decided to examine their plant's compressed air system
to determine whether any efficiency gains were possible.
The system review showed that the compressed air system
had two main problems that were leading to excessive
energy consumption.
The first problem was the compressor
control strategy. With the existing configuration,
each compressor's controls had to be manually set
with its own, independent control pressure band. Since
the plant's production characteristics are dynamic,
the air demand varies. As the air demand increased
and additional compressors were brought online, the
plant pressure fluctuated widely and the pressure
band between the first and the last compressor became
progressively wider. This pressure differential was
insufficient for a workable compressor control band,
leading the compressors to enter into each other's
throttling band. As a result, the plant could not
sequence the compressors appropriately and had to
operate all five of them continuously to ensure that
the volume and pressure needed during the demand peaks
would be met. All of the plant's compressors are centrifugal
compressors, which need to vent air when the system
demand falls below the compressors' minimum stable
flow (see text box). Analysis by plant personnel revealed
that approximately 14 percent of compressor output,
was being vented into the atmosphere.
Next, plant personnel identified
a significant pressure drop between the compressors
and the end-use applications, which explained pressure
fluctuations within the main header and inconsistent
pressure at the points of use. The main cause of the
plant's pressure drop was the leakage rate in the
plant's distribution piping network, which caused
the plant to set the compressor operating pressure
higher than necessary. This leakage rate, which was
almost 30 percent of the system's out-put, also created
artificial demand. Artificial demand is the excess
air required by a system's unregulated uses because
the system is being operated at a pressure level in
excess of actual production requirements. The leakage
rate required keeping all five compressors online
to ensure adequate air pressure for end-use applications.
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Centrifugal
Compressors
Centrifugal
compressors blow off, or vent, compressed air
into the atmosphere when the system demand falls
below the compressors' minimum stable flow.
This is because centrifugal compressors have
a limited throttling capacity. If the system
demand falls below a centrifugal compressor's
minimum stable flow, it will vent excess air
in order to prevent the system pressure from
rising above its set-point. If the system pressure
exceeds the compressor set-point, the airflow
can reverse direction and come back into the
impeller. This is a situation know as "surge"
and it causes serious damage to the compressor
requirements. The leakage rate required keeping
all five compressors online to ensure adequate
air pressure for end-use applications.
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PROJECT
IMPLEMENTATION
H.B. Reese is a subsidiary of the Hershey Foods Corporation,
producing confectionery products, such as chocolate-covered
peanut butter candies. The Reese plant in Hershey,
Pennsylvania, is a 47,000 square-meter facility with
900 employees. The facility doubled in size between
1957 and 1970 to accommodate the increase in demand
for its candy products. Compressed air is important
for the production process, because it is needed to
operate the cylinders on the wrapping machines and
robotic applications. These end-use applications require
very clean, moisture-free compressed air at a consistent
pressure level in order to operate reliably. Prior
to the project, the Reese plant had to operate their
compressors at a discharge pressure of 7.5 bar gauged
(barg) so that the robots and cylinders would receive
air at the minimum acceptable pressure level.
PROJECT
OVERVIEW
In 1995, the Reese plant commissioned
a professional survey of its compressed air system
because it was not able to generate compressed air
at the consistent pressure level needed for reliable
production. The survey led to a comprehensive strategy
to improve the plant's compressed air system's efficiency
and performance.
The survey found a number of issues
that prevented the plant's compressed air system from
operating optimally. As the plant's manufacturing
capacity had increased over time, additional compressors
were added. By the time of the survey, the plant had
a total of thirteen 75-hp rotary-screw compressors
housed in various areas of two buildings. Because
the buildings were not connected to each other, the
plant effectively had two compressed air systems that
were operating independently of each other. The survey
showed that if the two systems were to be connected
into one system, it would generate and deliver compressed
air more effectively.
Next, the survey found that the plant's
compressed air system experienced considerable pressure
loss between the compressors and the end-use applications.
This caused the system's pressure level to fluctuate
widely and led to inconsistent pressure at the points
of use. Pressure loss/drop is a function of a compressed
air system's dynamics-the interaction of airflow rate
with the inherent resistance of the pipeline and air
system components. The main factors that led to the
Reese plant's pressure drop were leaks in both the
plant's distribution piping network and components
in the system such as filters, regulators, lubricators,
hoses, and fittings. In addition, some of the components
were improperly sized for the airflow and pressure
that they were supposed to provide.
The leakage load at the time of the
assessment was 20 percent of the compressed air system's
out-put. Another 15 percent of the system's output
was being consumed unnecessarily in open blowing applications.
In addition, solenoid-operated condensate drains allowed
air to escape, causing the system pressure to fall.
The combination of leaks, inefficient components,
leaking condensate drains, and open blowing applications
caused the pressure level to fluctuate by 0.8 to 1.5
barg between the compressors and the production equipment.
When temporary demand events occurred, the pressure
level declined further and additional compressors
were brought online to bring it back up.
The factors that caused the plant's
pressure drop were also causing over 5,700 liter per
minute of artificial demand. Artificial demand is
the excess air required by a system's unregulated
uses because the system is being operated at a pressure
level in excess of actual production requirements.
In this case, the artificial demand that was created
by the leaks, undersized components, and unregulated
point of use operations required the system pressure
level to be set at a much higher level to maintain
minimum acceptable pressure for end-use applications.
Finally, the survey found that the
system's dryers were overloaded when the compressors
operated at high or low temperatures. This caused
some moisture to carry over into the system. The survey
recommended installing coalescing filters to mitigate
this situation.
PROJECT
IMPLEMENTATION
The central engineers concluded that
the system's control strategy was inefficient because
the individual, pneumatic controls on each of the
compressors did not allow them to react to the shifting
demand patterns in a timely way. The engineers decided
to replace the old controls with a single-loop, digitally
programmable logic control (PLC) system. The new control
system is capable of maintaining adequate pressure
differential between the compressor pressure settings
by centralizing the control of all five compressors
within one single-pressure band. The new controls
are sophisticated enough to respond to changes in
air demand and can sequence the compressors more efficiently.
In addition, the plant decided to identify and repair
air leaks twice a year during normal plant maintenance
shutdowns.
PROJECT
RESULTS
The installation of new and upgraded
controls has resulted in substantial energy savings
and improved efficiency of the plant's compressed
air system. The new controls provide a centralized
control strategy that is much more responsive to changes
in air demand, allowing the system to rapidly unload
compressors when they are not needed. This improved
control strategy has substantially reduced the compressor
blow-off rate and has, along with the leak repair
programme, allowed the plant to stabilize its header
pressure. The plant is now able to meet its air demand
with four compressors instead of five, leaving one
for backup use. The annual compressed air energy savings
are estimated at $75,000 (RM 285,000) and 2,143,000
kWh. With a project cost of $120,000 (RM 456,000),
the payback is estimated at one-and-a-half years.
LESSON
LEARNED
To gauge the success of a compressed
air system project, operating costs before the project
must be compared to operating costs after the project
has been completed. In any industrial facility, it
is important to benchmark the aggregate annual energy
costs as well as the costs per unit of production.
Once the improvements are made, the costs can be re-evaluated
to accurately measure the project's impact on the
plant's overall energy use as well as on its production.
By benchmarking and comparing its total energy costs
and its costs per unit of output, Michelin's Spartanburg
plant was able to obtain a more complete picture of
its project's success.
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